Researchers in California discovered that restricting water to their Early Girls improved the flavour of the tomatoes quite a bit, proving that environmental factor do make a difference. This technique is sometimes called "dry farming", and its proponents claim that it can make your average-flavoured hybrid taste as good as any heirloom tomato. However, giving your plants even and adequate water throughout the season will make the fruit larger and more plentiful, and the plants will be healthier. So the choice isn't much different than when choosing between hybrids and heirlooms: high yields, or excellent flavour?
Although this isn't scientific, here is my experience this year. I grew Early Girl tomatoes both in my backyard garden, which I water when it seems to need it, and a community garden bed, which has mostly had to fend for itself since we left on our vacation at the end of June. The vine-ripened tomato we ate today from the "dry-farmed" bed was firm and probably had somewhat better flavour than the tomatoes from our garden, but it was also quite small. The plants in the "dry-farmed" bed are also very puny compared to the ones we watered, and I know that in the end, we'll get many more times the tomatoes from the plants we watered. We have had a lot of cool weather this summer, which has not only made our tomatoes slow to ripen, but I think it may also have affected their flavour, which has been pretty mediocre.
A compromise between flavour and yield for anyone considering the "dry-farming" method might be to simply withhold water for a couple of days before the tomatoes are ready to harvest, hopefully improving the flavour without affecting the yield. If anyone tries this, please let me know if it works!